My 18 months female Jack Russel keeps weeing in the house- any where.. on the bed, floor, chair.
She knows its wrong.. she is a very happy energetic dog and I have had her from a pup... she knows outside is where she should go to wee.
She has also started to chew furniture, im at my tether, just dont know what to do anymore, I have started over and over to go back to training, but she keeps going back to being dirty.
Can anyone help ??

We need a bit more info I think
What is your existing house training protocol - what did you do to house train her?
How much exercise does she get?
How long is she left alone during the day?
Has she had a recent vet checkup?
Is she spayed?

House training started from day 1, letting her out regularly, treating and praising. I even have a training mat down by the back door, which she refuses to go on. She know to go out, but she will wee or poo where she stands when no one looks or is there, mostly upstairs. I have cleaned the carpets constantly so no smell remains. She is not left alone in the day. I have another dog who is fully trained. She goes out every day, and yes she is spayed. she has just had a vet check-up.. All clear. Her urine doesn’t smell only the usual.

If she is unreliable in the house she should never be anywhere where you can't see her. Until you are sure she is trustworthy she should always be in the same room with you on leash. This will be crucial for you to start re-building the house training.
If she is only messing when she is out of your sight it is because she has learned that it is rewarding to go in the house - as long as no one is looking. If you are punishing her for having accidents I would stop. Unless you catch her in the act the she will have no idea why she is being punished, and if you punish her for peeing/pooing in front of you she will only continue to do what she is already doing - make sure you don't see. When you do catch her having an accident you need to interrupt her. Clap once or twice and say "oops" or "outside" (loud enough to get her attention but not frighten her) and immediately lead her outside. Praise and reward heavily when she finishes.
Start taking her out for regular toilets and praise/treat her the second she's done. You may want to add in a game like fetch afterwards to increase the reward of successful elimination and to deal with some of the excess energy.
If she is never allowed to successfully eliminate in the house this behavior will extinguish itself.
Keeping her with you will also help you curb the chewing behavior. Start measuring out the food she would normally get in a day and put it into puzzle toys (kongs etc.) to engage her mind. Put something really smelly and delicious in the bottom, like wet food, to keep her interested. When you see her try to chew on something you don't want, re-direct her to one of her toys . You then need to make that object less desirable for her to chew in future. You can pick up a bitter spray from your local pet store - when her attention is diverted, spray the area thoroughly. This should make it a lot less rewarding
I know it seems like a lot but this will really help if done properly, and it's not for forever.

Let us know how it goes

(Oh, and make sure you are using with an enzymatic cleaner for her mess to de-nature the urine/fecal smell)

Is she crate trained? If not, would you be willing to go that route for when you have to leave her alone? If you place her in a crate when you are out and she gets to understand that the crate is a safe place (run a search on crate training to get some ideas on making the crate an enjoyable area) she is unlikely to mess in the crate and will not be able to get into trouble by chewing up the house. When you get home, first stop is to get her outside for a pee break at which time you would praise her.

The bitter sprays work well as does a small amount of alum mixed with water (teaspoon in a cup of water), unless you have one of those dogs that doesn't mind the taste of it (when we were training Lucy not to counter surf we tried leaving a piece of alum soaked bread on the edge of the counter, we heard her jump up to grab it and were expecting a spitting sound of her getting the bread out of her mouth only to find her happily eating it ).